Under the 1990 Clean Air Act Amendment, two regulated air pollutants are sulfur dioxide (SO.sub.2) and nitrogen oxides (NO.sub.x where x=1 or 2). Most of the SO.sub.2 control technology used in the United States is lime or limestone wet scrubbing. NO.sub.x currently cannot be removed effectively in these scrubbing systems due to the low solubility of nitric oxide (NO), the major constituent of NO.sub.x in combustion flue gases. Another major source of NO.sub.x gas is nitric acid pickling or etching, such as are used in the steel and semiconductor industries.
A number of methods of controlling NO.sub.x are known, but all suffer from a variety of drawbacks. Known methods include catalytic reduction with ammonia, scrubbing with NaOH, and scrubbing with NASH (sodium hydrosulfide). The principal drawbacks to catalytic NH.sub.3 reduction are extremely high capital costs and the required use of anhydrous NH.sub.3, a hazardous gas. Scrubbing with NaOH leads to the production of nitrates and nitrites, which must be removed and disposed of separately. Scrubbing with NaSH can lead to the production of reduced sulfur compounds, requiring separate removal disposal, and NaSH is itself classified as a hazardous chemical.
Rossi, in "How to Control NO.sub.x," Pollution Engineering, pp. 50-52 (April 1995), reported the use of an aqueous solution of H.sub.2 O.sub.2 for scrubbing NO.sub.x -containing off gas, and suggested the use of H.sub.2 O.sub.2 added directly to the process that uses nitric acid, such as in a stainless steel pickling process, so as to react instantaneously with the precursor of NO.sub.x, nitrous acid (HNO), before the HNO decomposes to NO and NO.sub.2. Kasper et al., in "Control of Nitrogen Oxide Emissions by Hydrogen Peroxide-Enhanced Gas-Phase Oxidation of Nitric Oxide," 46 J. Air & Waste Mgt. Assoc. 127 (1996), suggests gas phase oxidation of NO.sub.x to HNO.sub.3 at high temperatures by an acidic solution of H.sub.2 O.sub.2, followed by removal of the HNO.sub.3 in an alkaline solution. U.S. Pat. No. 5,397,549 discloses a process for removal of sulfur dioxide and nitrogen oxides from flue gases comprising cooling the flue gas, oxidizing the sulfur and nitrogen oxides to their highest valency states, and contacting the oxidized gas with calcium hydroxide and H.sub.2 O.sub.2 while maintaining the pH in the range of 4.5 to 5.5 to drive off CO.sub.2. U.S. Pat. No. 5,112,587 discloses a method for the removal of NO.sub.x comprising the gas phase catalytic treatment with H.sub.2 O.sub.2 to obtain HNO.sub.3.
There is still a need for a safe, inexpensive and efficient method of controlling nitrogen oxide emissions. This need and others are met by the present invention, which is summarized and described in detail below.